1. Field of the Invention
This invention relates to gasification of carbonaceous materials, and more particularly to apparatus for controlling bed height in fluidized bed gasification reactors.
2. Description of the Prior Art
In reactors for the gasification of carbonaceous materials, such as coal, a combustible product gas is produced, as well as solid waste products such as agglomerated ash. In the Process Development Unit (PDU) fluidized bed gasification reactor being operated for the United States Government, there is a single set of vertically positioned concentric injection tubes through which, in addition to particulate coal, various process mediums, such as recycled product gas, steam and oxygen, are injected. Additional fluidizing gas is injected through a sparger ring of circular cross section, concentrically disposed within the lower region of the gasifier vessel to provide a relatively balanced distribution of fluidizing gases across the vessel cross section. An imbalanced distribution can lead to channelling of the fluidizing gas flowing upwardly through the particulate matter. This can cause local slugging, excessive mixing, and local stagnation, as opposed to separation, of the char and ash particles.
In the PDU fluidized bed gasification reactor, feed particulated coal, in addition to producing a combustible product gas, intermediately forms char, and ultimately forms waste ash. The process takes place at temperatures in the range of 1400.degree. F. to 1900.degree. F., and above. The ash must be removed from the vessel, preferably continuously or by an on-line batch process, in order to maintain the process efficiently operational. It is desirable to remove only the ash as opposed to the incompletely reacted char, in order to maintain a high efficiency.
In order to achieve optimum gasification of the feed material it is necessary to provide sufficient residence time of the coal within the reactor vessel. Feed rates of the coal which are too high could force particles with residual gasification potential either down into the ash discharge (with a corresponding reduction in gas output) or up into the gas outlet (requiring additional gas cleanup to maintain gas quality). The effect of injection of too much or too little steam and/or combustion gas would have further detrimental effects on efficiency. What is needed is a method and apparatus for maintaining a material feed rate that is balanced with the gas and ash discharge rates. If the actual bed height of a fluidized bed is known, feedback can be provided to balance the input quantities with the output quantities. The method currently in use for determining bed height provides for a set of differential pressure (D/P) detectors with pressure taps at different elevations to be monitored as follows. A D/P detector with both taps in the gas region will show a differential pressure of 0 psi. However, a D/P detector with both taps in the bed region will show a pressure that is proportional to the weight of the bed contained between the two taps. Once this pressure is known, a D/P detector with one tap in the gas region and one tap in the bed region will show a pressure that is related to the weight of the bed between the two taps. It should be noted that the differential pressure shown will be very small. Over the entire height of the PDU reactor, about 31 feet, the differential pressure is only 1.8 psi. If the bed were completely homogeneous, the D/P detectors could be monitored continuously. Unfortunately, the bed contains numerous density anomalies in the form of pockets of gas, steam or coal, all of which effect the weight of the bed, but not necessarily the height of the bed. As a result, accurate control of the bed height by an automatic control is not possible using this bed height determination method because of rapid and random fluctuation of bed height indication.
What is needed then, is a method and apparatus for controlling the bed height by regulating material input rates, despite the presence of bed density anomalies which influence bed height determination.